Pathogenesis in Plant Pathology

• Introduction to Pathogenesis

Pathogenesis refers to the process by which a pathogen infects and causes disease in a plant. It includes the sequence of events from initial contact to the development of disease symptoms. Understanding pathogenesis is crucial for developing effective disease management strategies.

• Stages of Pathogenesis Plant pathogens follow a systematic sequence of infection that consists of the following stages:

1. Inoculation (Contact between Pathogen and Host)

• The first step where the pathogen comes in contact with the host plant.
• Inoculum: Any part of the pathogen (spores, bacteria, nematodes, viruses) that initiates infection.
• Sources of Inoculum:
  • Primary Inoculum: Comes from soil, seeds, crop debris, or infected plant material (causes primary infection).
  • Secondary Inoculum: Produced during the growing season and spreads the disease.
• Modes of Inoculum Dispersal:
  • Airborne: Puccinia graminis (Wheat rust)
  • Soilborne: Fusarium oxysporum (Wilt disease)
  • Waterborne: Pythium spp. (Damping-off)
  • Vector-borne: Xanthomonas campestris (Bacterial blight of rice, transmitted by insects)

2. Pre-Penetration (Germination and Recognition)

• The pathogen recognizes the host plant and prepares for penetration.
• Processes in Pre-Penetration:
  • Spore Germination: Fungal spores germinate on the leaf surface.
  • Attachment: Bacteria and fungi attach using adhesive substances.
  • Recognition Signals: Some pathogens identify their specific host using chemical signals.

3. Penetration (Entry into Host Tissue)

• Pathogens enter plant tissues through various mechanisms:
  1. Natural Openings:
     • Stomata: Puccinia spp. (Rust fungi)
     • Lenticels: Erwinia amylovora (Fire blight of apple)
  2. Wounds: Agrobacterium tumefaciens (Crown gall) enters through wounds.
  3. Direct Penetration:
     • Magnaporthe oryzae (Rice blast) uses appressoria to create pressure and break plant cells.
     • Rhizoctonia solani secretes enzymes to degrade cell walls.

4. Colonization (Invasion and Growth in the Host)

Once inside, the pathogen multiplies and spreads within the plant tissues. This phase determines the severity of the disease.

• Types of Colonization:
  • Intercellular (Between Cells): Xanthomonas spp. (Bacterial blight)
  • Intracellular (Inside Cells): Plasmodiophora brassicae (Clubroot disease)
  • Vascular System Colonization:
    • Xylem: Fusarium oxysporum (Wilt disease)
    • Phloem: Cucumber Mosaic Virus (CMV)
• Pathogens Use Different Strategies for Colonization:
  • Necrotrophs: Kill cells and feed on dead tissue (Botrytis cinerea – Grey mold).
  • Biotrophs: Keep host cells alive and extract nutrients (Puccinia spp. – Rust fungi).
  • Hemibiotrophs: Start as biotrophs and later become necrotrophic (Colletotrichum spp.).

5. Symptom Development (Expression of Disease)

• Visible changes in plants due to infection.
• Symptoms vary based on pathogen type:

6. Disease Progression and Systemic Infection

• Some pathogens remain localized, while others spread systematically through vascular tissues.
• Example:
  • Fusarium oxysporum spreads through xylem, causing wilting.
  • Potato virus Y spreads through phloem, causing mosaic symptoms.

7. Pathogen Reproduction (Multiplication and Dissemination)

• Pathogens reproduce and release new inoculum, spreading the disease.
• Reproduction Modes:
  • Fungal Spores: Sexual (Ascospores, Basidiospores) and Asexual (Conidia, Sporangia).
  • Bacterial Division: Binary fission.
  • Virus Replication: Uses host machinery to produce new virions.
  • Nematode Eggs: Hatch and infect new roots.

8. Pathogen Dissemination (Spread to New Hosts)

• Pathogens spread through:
  • Wind: Puccinia graminis (Wheat rust) spores travel long distances.
  • Water Splash: Phytophthora infestans (Late blight of potato).
  • Insects & Vectors: Tungro virus is transmitted by leafhoppers.
  • Seed Transmission: Xanthomonas campestris (Black rot of crucifers).

9. Survival (Dormancy and Overwintering)

• Pathogens survive in the absence of a host by forming resistant structures:
  • Fungal Pathogens: Chlamydospores (Fusarium oxysporum), Sclerotia (Sclerotinia sclerotiorum).
  • Bacteria: Survive in soil, seeds, or insect vectors.
  • Viruses: Persist in alternate hosts or vectors.
  • Nematodes: Survive as eggs in soil.

III. Host-Pathogen Interactions in Pathogenesis

• Susceptible Host: The pathogen completes its life cycle (disease occurs).
• Resistant Host: The host defends itself, preventing infection.
• Hypersensitive Response (HR): The plant kills infected cells to block pathogen spread.
• Systemic Acquired Resistance (SAR): The plant strengthens its defense mechanism.

1. Disease Management Strategies Based on Pathogenesis

• Avoidance: Crop rotation, resistant varieties, clean seeds.
• Exclusion: Quarantine, seed treatment.
• Eradication: Removal of diseased plants, soil fumigation.
• Protection: Fungicides, bactericides, insecticides.
• Host Resistance: Breeding disease-resistant varieties.
• Biological Control: Trichoderma spp., Bacillus subtilis suppress fungal pathogens.